1. Field of the Invention
This invention relates to the field of magnetic telegraphones, and more specifically to magnetic tape units employing one or more rotating heads which record and/or reproduce machine-convertible information while moving in transducing relationship with a magnetic web or tape. More specifically, the invention relates to means for controlling the magnetic web or tape to reduce interlayer slippage.
2. Prior Art
Rotating head magnetic tape units are widely known. In one form a generally cylindrical mandrel or drum includes a rotating head wheel which carries one or more read/write heads. The magnetic tape engages the mandrel at one point, makes a helical wrap about at least a portion of the mandrel and exits the mandrel at a point which is both axially and circumferentially spaced from the entrance point. The angle of helical tape wrap can vary in accordance with design choice, but is usually between 180.degree. and 360.degree.. The head wheel rotates so as to sweep its magnetic heads traversely across the tape. The angle at which the head enters and exits the tape may vary, in accordance with design choice, from slightly less than 90.degree. to a small angle, such as 15.degree..
Another form of device is one wherein the head wheel is associated with a tape guiding structure which bends the tape traversely into an arcuate shape that conforms to the circumferential shape of the headwheel. In this device, the tape travels in a generally straight line past the headwheel, and is traversely bent by the associated guides as it enters the headwheel area.
The present invention finds utility with either aforementioned type of device, and has been found particularly useful with the helically wrapped device.
Generally, the aforementioned devices are capable of operating in two modes. The so-called high speed mode of operation and the so-called stepping mode of operation. During the high speed mode of operation, the magnetic media is transported at a relatively high speed from a supply spool to a take-up spool or capstan. The high speed mode of operation is generally used to perform a search operation. During the stepping mode of operation the magnetic media is transported at a relatively low rate of speed from the supply spool to the take-up spool. In fact, the low speed mode of operation is, generally, functional when the device is stepping between adjacent tracks.
A major problem encountered in the aforementioned devices is that of interlayer slippage (i.e. layers of tape slipping relative to one another). Generally, the interlayer slippage is generated from the high speed mode of operation. As the flexible media is wrapped or coiled onto the take-up spool or capstan, air is entrapped between the layers of convolutions. The entrapped air forms layers of air film between the layers of tape on the capstan. Due to the air film between the tape layers, friction between adjacent layers of tape is substantially reduced which results in loosely wrapped-around tape.
During low speed operation the tape is stepped from one stripe to the next. The stepping is achieved by a torque which is supplied by the capstan motor to the capstan. However, after a high speed search, due to the air film which is entrapped between the tape layers, although the capstan moves as a result of the applied torque the torque is not transmitted to each individual layer of tape. As a result of the non-transmission of torque, the tape does not step or if the tape does step the direction of motion is opposite to the direction of capstan step. Due to the interlayer slippage an incorrect stripe will be in registry with the rotating head. As is well known in the art, each stripe on the media is generally identified by a unique identification number (ID). The system keeps a log or check on the ID of the stripe which the head must access after a step operation. When the system checks and finds that the head is now in registry with the wrong stripe, the system will ultimately reject the entire tape cartridge as defective when, in fact, it is not.
A more devastating result is that occassionally the head may record new information over the information which was previously recorded in the track and, therefore, destroy valuable customer data.
The prior art has adapted several approaches to solve the aforementioned problems, none of which are acceptable in a high speed data processing environment. In the first instance, prior art systems are designed to operate at an optimum speed during high speed search. The optimum speed is selected so that air will not be entrapped between the layers of the media; or if air is entrapped only a relatively small volume which will not aggravate the interlayer slippage problem. By designing the prior art devices to operate at an optimum speed, the processing time of the overall system is significantly increased. As is well known, the current trend in high speed data processing systems is to minimize the processing time (that is time required to access information from the storage means). As such, the first approach practiced in the prior art, to solve interlayer slippage, is not acceptable.
In a second attempt to solve the interlayer slippage problem, the prior art devices are designed with a squeegee means. The squeegee means is positioned relative to the capstan on which the tape is wound. By forcing the squeegee to contact the surface of the wound tape, entrapped air is squeezed out from between the tape layers.
Although this approach is an improvement over the first aforementioned discussed solution, it suffers from several drawbacks. In order to control the contact between the take-up spool and the squeegee, both mechanical and electrical means are required for controlling the positioning of the squeegee. For example, during high speed mode of operation the squeegee cannot be in contact with either the capstan or the tape which is wound on the capstan. At the end of a high speed search the squeegee has to be forced in contact with the media or tape on the capstan. The mechanical linkage and electrical circuit which is required to control the squeegee tends to increase the overall cost of the device. Additionally, the time required for the mechanical linkage to react tends to lower the accessing time for the device. A more detailed discussion of the squeegee system is given in U.S. Pat. No. 3,309,037 wherein a squeegee means is used to squeeze out air entrapped between layers of magnetic media.
Another problem which the squeegee solution introduced is that of unusual tape wear. Due to the fact that the squeegee is in contact with the oxide surface of the media, scratches are often transferred to the media. Due to the scratches, the useful life of the media is significantly reduced.
Still another approach which is practiced in the prior art to solve interlayer slippage is backcoat roughness or substrate roughness. In this approach the backcoat or substrate of the media is roughened beyond conventional limits. The particles which are placed on the backcoat so as to create the roughened surface project into the space between adjacent layers and inhibit the formation of air film.
However, backcoat roughness as a solution to the interlayer slippage problem is effective only when the roughness is at an optimum value. If the roughness falls below the optimum value its effect in solving the interlayer slippage problem is negligible. Media whose backcoat has a roughness which is equal to or greater than the optimum range is plagued with the phenomenon known in the art as "print through". "Print through" is the phenomenon wherein adjacent layers of media are embossed as a result of the particle size which is applied to the backcoat of the media to cause its roughness. "Print through" affects the density at which data is recorded on the oxide surface of the media. This is so because as a result of the embossing the oxide surface of the media is no longer smooth; hence the magnetic transducer does not fly at a uniform height and/or relatively close to the oxide surface of the media. As is well known to those skilled in the art it is possible to have denser recording the closer a magnetic transducer flys to the oxide surface of the media.
Another problem which the "print through" creates is an increase in the frequency of head/media crashes which result in unusual head and tape wear. As the tape is embossed by the "print through" phenomenon the oxide surface of the media is covered with a plurality of mounds or projections which project above the oxide surface of the tape. As the magnetic transducer flys over the media which has a non-uniform oxide surface the transducer crashes into the mounds.